who in riverdale would be the most compelling main character, other than archie- and why?
This question is Hard Level. Maximum, even. OMG?
The reasons why: Cheryl does things. She wants things desperately, she acts out of passion, she goes all out, she has all the feelings ever felt by anyone happening to her all at the same time. She gets the superpower of slo-mo that the others of the Core 4 usually only get in moments of extreme distress. She also has a LOT of storylines in which she already is the protagonist - the Gothic Blossom incest drama, the mean high school cheerleader narrative, the poor-little-rich-girl narrative, the martyred lesbian trapped in a homophobic world narrative, the hermit-artist-witch narrative, and so on and so on.
It's also as a result of the process of elimination, honestly. Betty is as much put-upon as Cheryl, but she's actually much more inhibited and never quite breaks out of that inhibition. Like, if Cheryl was to find out she's the Whore of Babylon, right, she wouldn't just get all glum, sad and bitter like Betty did. She'd go do a rite to summon all the other Whores of Babylon and make a cheerleading team for Lucifer and set heaven on fire or whatever. Veronica is my beloved, but she does not have enough there to be main character - her issues are all relational: as daughter/sister to Hiram, as daughter/friend to Hermione, as sister/friend to Hermosa, as friend/lover to Betty, Archie, Jughead and Reggie. She's kind of the Nick Carroway to everyone else's Jay Gatsby. Reggie is comic relief when he's not unreliable villain. Everyone else is too underdeveloped to take center stage (Sorry, Kevin).
And finally, I would argue that Jughead constantly invades the "main character" arena that Archie is supposedly occupying throughout the canon of the show, so I'm not at all sure actually that Archie is the main character. The story always reveals more about the author than anything else, right?